1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus utilizing a recording medium formed of a photorefractive material, i.e. so-called a holographic memory.
2. Description of the Related Art
A holographic memory system is known as a digital information recording system which applies the principle of holography. The information recording system is characterized by recording information signals recorded on a recording medium as optical signals. For the recording medium, a photorefractive crystal such as lithium niobate single crystals are used.
There is a conventional holographic recording and reproducing method utilizing the Fourier transform.
FIG. 1 shows a conventional 4f-based holographic recording and reproducing apparatus. A laser light beam 12 emitted from a laser light source 11 is split into a signal light beam 12a and a reference light beam 12b in a beam splitter 13. The signal light beam 12a is expanded in its diameter by a beam expander 14 as a collimated light beam, and then irradiated to a spatial light modulator (hereinafter abbreviated as “SLM”) 15 including a dot matrix panel such as a transmission-type TFT liquid crystal display (LCD) panel to which image data to be recorded are provided which are converted by an encoder as electric signals. Thus, the panel forms a bright and dark dot pattern on its plane corresponding to the image data. The signal light beam 12a is optically converted by the SLM 15 to include data signal components. The signal light beam 12a including dot pattern signal components passes through a Fourier transforming lens 16 which is positioned at a focal distance f apart from the SLM 15. The Fourier transforming lens 16 performs Fourier transformation and then the signal light beam 12a including dot pattern signal components is converged into a recording medium 10. On the other hand, the reference light beam 12b split from the beam splitter 13 is guided to the recording medium 10 by a fixed mirror 17 and a rotary mirror 17a, and intersects an optical path of the signal light beam 12a within the recording medium 10 to form a light interference pattern. The recording medium 10 made of a photorefractive crystal records the spatial intensity modulation represented by the light intensity of the light interference pattern as changes in refractive index.
In the foregoing manner, the diffraction light from the image data illuminated by a coherent collimated light is focused through the Fourier transforming lens 16 and changed into a distribution on the focal plane, or Fourier plane. The distribution as a result of Fourier transformation is interfered with the coherent reference light to record an interference fringe thereof to the recording medium placed in the vicinity of the focal point. Ending the record of the first page, the rotary mirror 17a is rotated a predetermined amount and parallel moved in position a predetermined amount so that the incident angle of the recording reference light beam 12b on the recording medium 10 is changed to record the second page by the same procedure. In this way, the angle-multiplexed recording is carried out with sequential recording as the above.
In reproducing information, on the other hand, inverse Fourier transformation is carried out to reproduce a dot-pattern image. As shown in FIG. 1 the optical path of the signal light beam 12a is cut off, for example, by the SLM 15 to illuminate only the recording reference light beam 12b to the recording medium 10. In order to make incident the recording reference light beam 12b on the medium at the same angle as the recording reference light of upon recording the page to be reproduced, the rotary mirror 17a is changed and controlled in position and angle by the combination of mirror rotation and parallel movement. Reproductive light of the recorded interference pattern appears at an opposite side of the recording medium 10 to the side illuminated by the signal light beam 12a. If the reproduced light is guided to and inverse-Fourier-transformed by an inverse Fourier transforming lens 16a, the dot-pattern signal can be reproduced. Furthermore, if the inverse Fourier transforming lens 16a images the dot-pattern signal on an imaging device or photodetector 20 using a CCD (Charge Coupled Device) or CMOS sensor arranged in the focal point, and reconverted into an electric digital data signal and then sent to a decoder, the original data is reproduced.
In this manner, the conventional apparatus requires a high-performance Fourier transforming lens and inverse Fourier transforming lens. Accordingly, there is a problem with the disadvantage for system size reduction.
Meanwhile, there is a reproducing method with a phase conjugation wave as one of the methods of reducing the size of a hologram memory system. In order to realize a reproducing method with a phase conjugation wave, a reference light upon recording (described as reproducing reference light) that is phase-conjugative to the reference light upon recording (described as recording reference light) can be generated by a phase conjugation mirror. However, it is not easy to realize such a phase conjugation mirror.